A circle system is used to ventilate patients undergoing general anesthesia. To minimize wastage of excess expired anesthetic breathed out by the patient, the circle breathing system is designed to enable patient expired gases to be rebreathed after carbon dioxide is removed using CO2 absorbent. In addition, oxygen and anesthetic agent is replenished to maintain desired concentration of gases breathed by the patient. CO2 absorbent housed in a canister has a finite capacity to remove CO2 from the expired patient gases. They can be replaced at the start of day or end of day on a routine basis. This is wasteful as unused absorbent capacity is discarded.
Alternatively, the absorbent is replaced during an anesthesia case when it is spent. This is detected by measurement of significant inspired CO2 concentration. A typical threshold value is 0.5% of sustained inspired CO2 concentration. This cost saving practice exposes the patient while unconscious and requires mechanical ventilation assistance during anesthesia, where the risk is disruption of ventilation that include temporarily pausing ventilation, disconnecting the breathing system, installing a CO2 canister with fresh absorbent, checking the integrity of the reconnected breathing system, and resuming ventilation.
Dye with color changes in the presence of CO2 is also used to indicate sent absorbent, as is computation of remaining CO2 absorption capacity based on the absorbent refilled quantity and rate of CO2 recirculated. Since quantity of refill and efficiency of the packed absorbent is a poor estimate of usable absorbent, the estimator/gauge is inaccurate.